I am copying my root directory to a ZFS pool. One thing I have noticed while rsyncing is that the Size column in df -h dereases for all but one of my data sets, even though they are all in the same pool, for example, df -h shows:

I would think that the Size in the left column would remain constant, the used would differ, and then Avail would just be Size-sum(used). Maybe I am just missing something about how ZFS reports disk space? Only one (rpool/HOME/dayd) seems to behave somewhat like I would expect.